An Impala, 2008, 3.5L is making creaking and gurgling sounds after a drive. When I shut the car off, the noise begins withing the first 30 seconds to a minute. There are tiny bubbles streaming into the coolant reservoir from the overflow tube. The coolant level is not low. The car does not overheat, There is no evidence of coolant in the crankcase or oil in the cooling system. I should mention that this is intermittent. I suspect it will be more constant as the weather gets warmer. Do I have a head gasket leak, a bad radiator cap, a thermostat problem?
Keep a close eye on coolant level. You might have a head gasket getting ready to go.
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You can get a cheap head gasket leak tester that will tell you. It draws air from the radiator through a liquid that changes color if combustion byproducts are in it.
Just make sure you do it right, because if you draw any coolant into the test vessel, the test liquid will also change color and make you think you have a head gasket leak. 
The cooling system cap may not be holding pressure. After turning off the engine squeeze the upper radiator hose, it should be hard like an inflated bicycle tire. If the hose collapses with no resistance the cap is not holding pressure, this causes boiling after shut off.
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I considered getting this sort of a device at Harbor Freight. The reservoir is not pressuried; do you think that woujld be enough to sense the presence of combustion gases?
Sometimes, it feels squishy, like there is nothing in it. Thanks.
Yes, it’ll work if you follow the instructions. You do it in the radiator, not the reservoir, and you hook the tester up to engine vacuum which draws air through the tester. It doesn’t require pressurization in the coolant system (which is good, 'cause the radiator cap is off!). Obviously, do not open the radiator cap unless the engine is cold.
Next step, replace the radiator cap.
I would also replace the thermostat at this age.
I would also use only OEM parts, not aftermarket.
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Cleaning the rubber gasket on the cap and the mating surface on the radiator may make a difference. The sealing is a function of that interface, plus the spring tension the cap provides.
I agree to use OEM parts. I wouldn’t disturb the thermostat unless there was evidence of a problem with it.
Good suggestion on cleaning the gasket on the radiator cap. It seems like a good place to start in the process of eliminating possibilities. Thanks.
Never used one of these. Thanks.
So I resist the idea that things fix themselves. This bubbling went on for several weeks during the spring and I anticipated it would get worse with the increasing heat of summer. Sub . . . it hasn’t done this for several weeks now. I have never figured out what the problems is that caused the bubbling. So, I don’t know what is now different. I only know that there is still no evidence of cross contamination between engine oil and coolant.
Sometimes very small leaks can be “fixed” by getting plugged up by stuff floating in the fluid. I had a minor exhaust header leak in an old truck that I never fixed because the header wasn’t available anymore, and I didn’t feel like hunting down someone willing and able to weld rusty old cast iron. When the leak manifested you’d hear this, quite literally, farting noise coming from the header as gasses escaped.
But the vehicle also ran somewhat rich, so oftentimes soot would plug up the leak and then the truck wouldn’t fart anymore until either I ran seafoam through the engine, or for whatever reason drove it hard, which would dislodge the plug and then I had a farting truck again for awhile until soot patched it for me again.
The leak “fixed itself” without actually fixing the problem.
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A new cooling system cap is $10.
If you overheat the engine severe enough to get coolant in the oil, that will be a $4000 repair.
I’ll second the idea of trying a new radiator cap (or pressure cap, depending on how the system is configured). I hear a gurgling sound when I’m in the process of removing the radiator cap on my truck after the engine has cooled. If the cap seal were leaking it seems very possible it would make that sound on its own as the engine cooled.
If you decide to replace the thermostat, make sure you know which way it came out so you can put it back in the same way, some thermostats can be put in backwards and the thermocouple (the mechanism that opens the thermostat) would be on the wrong side to pick up the coolant’s temp and may not open at all…
Since most folk have smart phones, I suggest you also snap a photo before you start, in case you need to move wires or something, and snap a photo of the thermostat as it originally sat in its housing. Besides helping to keep you from making simple mistakes, it makes great Facebook photos to show all your couch potato friends what you did so they can live vicariously through your “exploits…”
Yeah, I know it’s near impossible to put it in backwards, but who puts gas in a diesel, diesel in a gas auto (using the oil filler funnel because the diesel pump’s nozzle will not fit), oil in the radiator, coolant into the crankcase, oil in the brake reservoir, or brings a Tesla to a gas station to fill it up, etc…
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All true bjensky! Any of the aforementioned flubs can happen - and do.